The long-term goal of this research is to understand the cellular and molecular mechanisms of cell-cell communication between a motor neuron and muscle fiber during the formation of a functional synapse. Specifically, we want to address how motor neurons are remodeled during the development of an adult nervous system. We use the (adult) Dorsal Longitudinal (flight) Muscle synapse of Drosophila as a model. We previously showed that the basic neuromuscular pattern of the DLMs develops during the first day of metamorphosis, and described the remodeling of persistent larval motor neurons to innervate adult muscle targets. This proposal focuses on the remaining 3 days of the pupal phase, and examines how adult-specific motor neuronal arbors are pruned back and stabilized to give rise to the mature adult DLM synapse. The synaptic plasticity evident during this process is more pronounced than changes such as those occurring during larval synaptic growth, and bears striking similarities to synapse refinement seen in vertebrate nervous systems. We hope to demonstrate that the (adult) DLM synapse has many characteristics of its vertebrate counterpart. Three main objectives are presented: a) Identify the precise time period during which pruning of peripheral motor neuronal arbors occurs. b) Use genetic and electrophysiological approaches to examine the role of electrical activity in pruning. c) Follow the maturation of neuromuscular contacts into functional synapses using synapse-specific markers and ultrastructural approaches. Most of the studies on synapse formation and maturation in Drosophila have focused on the embryonic and the larval system while development of the adult synapse remains to be explored in the same detail. Since persistent larval motor neurons are respecified to become adult motor neurons, our studies of pruning during metamorphosis will describe an additional level of synaptic plasticity of these motor neurons that occurs during the adult (second) phase of synaptogenesis. Embryonic and adult NMJ formation have previously been studied in isolation. Our studies will serve to strengthen the continuity between the two remarkably distinct stages in the Drosophila life cycle, and serve as a basis for further studies on understanding the molecular bases of the differences and similarities with respect to synaptogenesis. In doing so, we hope to better understand the nature of cell-communication between pre- and post-synaptic cells (retrograde and anterograde signaling) that is central to the manifestation of synaptic plasticity. These studies will have implications for better understanding the neurodevelopmental basis of diseases such as epilepsy, schizophrenia and Rett's syndrome, where pruning is thought to be defective.